Method for producing glass preform for optical fiber

ABSTRACT

A method for producing a glass preform for use in the fabrication of an optical fiber, which includes the steps of forming a glass soot preform from a glass-forming raw material and heating the soot preform in an atmosphere containing SiF 4  under a pressure higher than 2 atm. for a period of time sufficient to add fluorine during the time between the formation of the soot preform and the vitrification of it, fluorine is added at a high rate and in a larger amount.

This is a divisional application of Ser. No. 526,522 filed May 21, 1990,now U.S. Pat. No. 5,053,068, which is a continuation of Ser. No.211,158, filed Jun. 22, 1988, now abandoned, which is a continuation ofSer. No. 855,396, filed Apr. 24, 1986, now abandoned.

FIELD OF THE INVENTION

The present invention relates to a method for producing a glass preformfor use in the fabrication of an optical fiber. More particularly, itrelates to a method for producing a glass preform for use in thefabrication of an optical fiber containing fluorine as an additive.

BACKGROUND OF THE INVENTION

A glass preform for use in the fabrication of an optical fiber isproduced by various methods. Among them, the VAD method and the OVPDmethod are attractive methods since their productivity and quality ofthe fabricated optical fiber are better than other methods. Thesemethods comprise synthesizing glass soot particles by flame hydrolysisof a glass-forming raw material and depositing the soot particles on arotating seed material to form a porous soot preform. Then, the poroussoot preform is heated in a suitable atmosphere containing variouscomponents to dehydrate and vitrify it so as to produce a transparentglass preform, which is drawn to fabricate an optical fiber.

The optical fiber comprises a core through which light is propagated anda cladding which surrounds the core and reflects light to be propagated.Numerical aperture (hereinafter referred to as "N.A.") is calculatedfrom the average refractive indexes n₁ and n₂ of the core and thecladding as follows:

    N.A.=√n.sub.1.sup.2 -n.sub.2.sup.2 (n.sub.1 >n.sub.2).

It is understood that the difference of the refractive index between thecore and the cladding is made large to obtain a large N.A. To this end,one of following measures is taken in the case of a silica (SiO₂) glasstype optical fiber:

1) In the core, an additive for increasing its refractive index isadded.

2) In the cladding, an additive for lowering its refractive index isadded.

3) Combination of the measures 1 and 2. Needless to say, the cladding inthe case 1 and the core in the case 2 are made of silica glass.

Usually, GeO₂, P₂ O₅, Al₂ O₃ and TiO₂ are used to increase therefractive index of silica glass, and B₂ O₃ and fluorine (F) are used tolower the refractive index of silica glass. FIG. 1 shows the variationof the refractive index of silica glass added with the various additivesfor light with a wavelength of 0.59 μm (cf. Kumamaru and Kurosaki,"Light Transmission Materials" in the Industrial Materials(Kogyozairyo), 27 (1979) 39).

Among the additives, fluorine has recently gained the attention of thoseskilled persons in this field and is being studied as an additive in theVAD method and the like.

To achieve the same difference of refractive index between the core andthe clodding of the optical fiber, the measure 2 or 3 is preferred sinceany additive is not added to the core, or a smaller amount of theadditive is added to the core than in the measure 1. This isadvantageous for a high N.A. optical fiber since attenuation of lighttransmission due to the presence of the additive is decreased. Inaddition, an optical fiber having good performance in the presence ofradiation can be produced only by the measure 2. Therefore, it isgreatly advantageous to add the additive to the cladding so as to lowerits refractive index.

In the VAD method, fluorine is added in a sintering step, namely avitrifying step of the soot preform. This has the following advantages:

1. Fluorine is homogeneously added to achieve uniform distribution ofthe refractive index, and

2. The addition rate of fluorine is high. Namely, several to tens ofhundreds of grams of the porous soot preform can be treated andvitrified within several hours.

In the conventional methods, even when the soot preform is heated underatmospheric pressure in an atmosphere of a 100% pure fluorine-containingcompound in order to add fluorine to the preform, fluorine is added atmost in an amount corresponding to -0.75% of the refractive indexdifference.

By other methods, for example, a plasma outside deposition method, theglass-forming raw material is deposited on the seed material by means offlame generated by plasma so as to directly vitrify the raw material onthe seed material. If the fluorine-containing compound is added in theglass-forming material to add fluorine in the glass in an amountcorresponding to -1% of the refractive index difference, the depositionrate of the glass is at most 0.1 g/min. and lowered as the amount of theadditive is increased.

However, according to the conventional methods, including the VAD methodand the plasma method, bubbles tend to undesirably remain in the glasspreform when fluorine is added in an amount corresponding to -0.5% ofthe refractive index difference. The amount of the bubbles increases asthe added amount of fluorine increases.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a method for producinga glass preform containing fluorine in an increased amount with fewer orno bubbles therein.

Another object of the present invention is to provide a method forproducing a glass preform by which fluorine is added at a high rate.

These and other objects are accomplished by a method for producing aglass preform for use in the fabrication of an optical fiber accordingto the present invention, which comprises the steps of forming a glasssoot preform from a glass-forming raw material and heating the sootpreform to vitrify it, wherein the soot preform is heated in anatmosphere comprising SiF₄ under pressure higher than 1 atm. for aperiod of time sufficient to add fluorine between the formation of thesoot preform and the vitrification of it.

Preferably, SiF₄ is flowed in the atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing variation of the refractive index of silicaglass added with the various additives for light with a wavelength of0.59 μm,

FIG. 2 is a graph showing a relationship between the partial pressure ofSiF₄ and the difference of the refractive index from that of puresilica,

FIG. 3 is a graph showing a relationship between the heating temperatureand the difference of the refractive index, and

FIGS. 4 and 5 schematically show preferred embodiments of the apparatusfor carrying out the method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

It may be easily assumed that the reaction rate is increased by heatingthe porous soot preform in an atmosphere of the fluorine-containingcompound under elevated pressure. However, by simply heating the porouspreform in a sealed vessel pressurized with the fluorine-containingcompound, any glass preform having good quality cannot be produced. Thisis partly because heavy metals from the vessel wall would contaminatethe preform, and partly because the compound would thermally decomposeso that the reaction efficiency is decreased.

In addition, any component other than fluorine such as carbon from CF₄and sulfur from SF₆ may be contained and form bubbles in the glass. Thisis confirmed by the fact that the bubbles contain CO₂ and/or CO when CF₄is used to add fluorine in the glass preform.

Since SiF₄ is used as the fluorine-containing compound to add fluorinein the glass preform according to the present invention, it reacts withquartz glass according to the following reaction formula (1)

    3SiO.sub.2 (s)+SiF.sub.4 (g)→4SiO.sub.1.5 F(s)      (1)

wherein (s) and (g) stand for solid and gaseous states, respectively.Therefore, different from the conventional method utilizing CF₄ or C₂ F₆to add fluorine, any undesirable gas such as CO₂ or CO is not formed.

In addition, when SiF₄ is continuously flowed under elevated pressure,the contaminating materials from the vessel wall are removed before theyreach the preform. Thereby, the surface of the preform can be keptclean.

By continuously providing fresh SiF₄ in the vessel an, optimum reactionrate can be maintained. This may be due to the fact that thedissociation reaction represented by the following reaction formula (2)could be suppressed:

    SiF.sub.4 (g)→SiF.sub.2 (g)+F.sub.2 (g)             (2)

When SiF₄ is continuously flowed its flowing rate depends on otherconditions such as the size of the heating vessel, the heatingtemperature and the like. Usually, it is not lower than 50 ml/min.,preferably 100 to 500 ml/min.

When the porous soot preform produced by the VAD method is heated at1,200° C. for 3 hours, the relationship between the partial pressure (P)of SiF₄ and the difference of the refractive index (Δn) from that ofpure silica is shown in FIG. 2. Under the higher pressure, fluorine ismore effectively added in a larger amount so that the refractive indexbecomes lower.

The relationship between the heating temperature (T° C.) and Δn is shownin FIG. 3. As the temperature is raised and the partial pressure of SiF₄is increased, the difference of the refractive index is increased.Practically, when the partial pressure of SiF₄ is higher than 20 atm.,or when the temperature exceeds 1,400° C., the bubbles tend to form inthe glass preform. On the contrary, when the temperature is too low, thereaction does not completely proceed. Therefore, the temperature ispreferably not lower than 800° C.

Preferred embodiments of an apparatus for carrying out the method of thepresent invention are schematically shown in FIGS. 4 and 5, whichinclude a supporting rod 1, a porous soot preform 2, a pressure vessel(furnace) 3, heating sections 5 and 7 and a gas supply system 8. Theapparatus of FIG. 5 further comprises a pressure gauge 9, an outlet ofgas 10 and a valve 11.

Practical and presently preferred embodiments of the present inventionare shown in the following examples.

EXAMPLE 1

In the apparatus of FIG. 4, a pure silica soot preform was heated in thevessel 3 containing pure SiF₄ under pressure of 4 atm. at 1,100° C. for2 hours to melt and vitrify it. The difference in the refractive index(Δn) of the fluorine-added glass preform was -1% in comparison with puresilica.

The glass preform was jacketed by a quartz tube and drawn to fabricatean optical fiber, which contained few impurities and had a lowattenuation of light transmission of 2 dB/km at a wavelength of 0.85 μm.

EXAMPLE 2

In the apparatus of FIG. 5, a soot preform comprising a silica glasscore and porous silica glass deposited around the core was heated at1,100° C. under 2 atm. for one hour with flowing SiF₄ at a rate of21/min. Then, the preform was vitrified at 1,600° C. in an atmospheremainly containing helium. The difference of the refractive index betweenthe core and the cladding was 0.4%.

EXAMPLE 3

In the apparatus of FIG. 4, a preform comprising a core made of a GeO₂added high N.A. glass with Δn of 2% and porous silica deposited aroundthe core was heated at 1,350° C. under 5 atm. for one hour with SiF₄flowing at a rate of 50 ml/min. and vitrified at 1,800° C. under 1 atm.in helium atmosphere. The produced preform had a large N.A. with a Δn of-2% in the cladding portion and about 4% in the core portion.

According to the method of the present invention, glass having a |Δn|larger than 1% can be produced. Fluorine is added at a high rate. WhenSiF₄ is continuously flowed during the addition of fluorine to theporous soot preform, the reaction rate of fluorine is not decreased andless bubbles are formed in the preform. Further, a glass preform with alow Δn of the cladding is easily formed. Such preform is advantageousfor the fabrication of a high N.A. optical fiber or an optical fibercomprising a pure silica core. By the method of the present invention,contamination of the glass preform from the wall of the heating vessel(e.g. heavy metals and hydroxyl groups) can be suppressed or prevented.

What is claimed is:
 1. A method for producing a silica glass preformcontaining fluorine for use in the fabrication of an optical fiber,which comprises the steps of:(1) forming a porous silica glass sootaround a transparent glass rod core from a glass-forming raw material byvapor deposition to obtain a complex glass preform, (2) heating thecomplex glass preform in an atmosphere consisting of SiF₄ under apressure of from 2 atm. to 20 atm. for a period of time sufficient toadd fluorine to the complex glass preform during the time betweenformation of soot preform and a subsequent vitrification of the preform,and (3) heating the complex glass preform formed in step (2) at a highertemperature at which the complex glass preform is vitrified to form atransparent silica glass preform.
 2. The method according to claim 1,wherein SiF₄ is continuously flowed in the heating atmosphere.
 3. Themethod according to claim 2, wherein SiF₄ is flown at a rate not lowerthan 50 ml/min.
 4. The method according to claim 1, wherein the heatingtemperatures are not lower than 800° C.
 5. The method according to claim4, wherein the temperatures are not lower than 800° C. and not higherthan 1,400° C.